The p53 tumor suppressor is mutated in at least half of all human cancers, and mice deficient in p53 develop cancer at 100 percent frequency, underscoring the critical role of p53 in preventing tumorigenesis. p53 exerts its tumor suppressive function by sensing cellular stress and inducing cells to undergo either G1 arrest or apoptosis to limit their proliferation. Although the mechanism by which p53 activates G1 arrest is becoming clearer, there is considerably less understanding of the mechanism by which it induces apoptosis. To identify genes specifically activated during apoptosis, we performed a subtractive hybridization screen in which G1-arrested cell RNA populations were subtracted from apoptotic cell RNA populations. In this screen, we isolated a novel gene, PERP (p53 apoptosis effector related to PMP-22), that is preferentially expressed in apoptotic cells. PERP encodes a novel tetraspan membrane protein related to the PMP-22/gas3 protein commonly implicated in human hereditary peripheral neuropathies such as Charcot Marie Tooth disease. PERP is induced during p53-dependent but not p53- independent apoptosis, and expression of PERP in cells is sufficient to induce cell death, together suggesting it is a strong candidate mediator of p53 function in apoptosis. The role of PERP in p53 function will be investigated in this proposal. To conclusively determine the importance of PERP in p53 function, a PERP knockout mouse has been generated. Through analysis of various cell types derived from PERP-deficient mice, the importance of PERP for p53-mediated apoptosis will be defined. Furthermore, by studying PERP null mice, the role of PERP in normal development as well as in p53-mediated tumor suppression will be elucidated. If there is an embryonic lethal phenotype of the PERP null mice, we will utilize a conditional PERP knockout mouse we have also generated to specifically inactivate PERP in certain tissues and determine its role as a tumor suppressor in those contexts. In addition, the mechanism by which PERP induces cell death will be examined through cell biological approaches, to clarify the link between p53 and the apoptotic machinery. Together, these approaches will define the role of PERP in mediating p53 function in apoptosis and tumor suppression. As activation of the p53-dependent apoptotic response is thought to determine the response of at least some tumor types to radiotherapy and chemotherapy, an understanding of the pathway also has important clinical implications, both for prognosis and therapy.